Field of the Invention
The present invention relates to a process for treating offgases and an integrated urea melamine plant.
The present invention relates to a method for treating offgases which are obtained in a melamine synthesis reactor which is part of an integrated process for melamine and urea production.
Description of Related Art
Integrated processes for urea and melamine production have been known, wherein melamine is produced in a melamine plant, in particular in a melamine synthesis process of the low pressure type or high pressure type. Melamine synthesis uses ammonia and urea as raw material, wherein the urea is being produced in a urea production plant, to which the offgases coming from the melamine synthesis plant or reactor which substantially contain ammonia and carbon dioxide are recycled as raw materials.
In case of high pressure processes for the synthesis of melamine urea and ammonia are reacted at temperatures between 320 and 450° C. and pressures between 5 to 60 MPa whereby liquid melamine and offgases, which mainly consist of ammonia, carbon dioxide and low amounts of gaseous melamine are obtained. After separating the melamine melt from the offgases said melamine melt is being processed using different methods for obtaining pure melamine. The offgases are preferably recycled to a urea plant.
Before recycling the offgases to the melamine plant said offgases have to be treated in order to remove the gaseous melamine and further side products, since said gaseous melamine and side products can hamper the urea production.
Different methods for recycling the offgases to the urea plant comprising the step of removing gaseous melamine and its side products from the offgases before recycling the offgas to the urea section are known.
According to U.S. Pat. No. 3,723,430 the offgases leaving the high pressure melamine synthesis reactor enter an offgas washing unit, wherein the offgases are washed or scrubbed with urea melt. The urea leaving the washing section contains now the remaining melamine and side product and is fed directly to the melamine synthesis reactor. The purified or scrubbed melamine offgases are subsequently directly transferred into a low pressure urea reactor without condensation.
WO 2005/080321 A1 relates to an integrated process for urea and melamine production wherein the offgases resulting as by-products of the melamine synthesis are washed or scrubbed with urea solution or melt in a washing unit or scrubber unit and are subsequently fed to an offgas condensation section. In said condensation section the offgases coming from the melamine synthesis section and a carbamate aqueous solution are mixed together and condensed. The carbamate solution stems from the urea recovery section of the urea plant. In the condenser unit the offgases are completely condensed in the carbamate solution by indirect heat exchange with a cooling fluid, such as water, so that a concentrated carbamate aqueous solution is obtained which is subsequently fed back to the urea synthesis section.
WO 2008/052640 A1 relates also to an integrated process for urea and melamine production, wherein the urea plant is of the total recycle type and the melamine plant is a high pressure plant. The offgases of the melamine synthesis are scrubbed or washed with liquid urea in a scrubber unit and are obtained at a pressure of at least 0.2 MPa, usually with a pressure of 1 to 7 MPa. The condensation of the offgases takes place in the medium pressure section of the urea plant, wherein the condensation is carried out using an aqueous carbamate solution recycled from the urea synthesis. The medium pressure section of the urea plant has the same pressure as the offgases from the melamine plant. Therefore, the condensation takes place at the pressure of the offgases. The aqueous carbamate solution used for condensation is only mixed with the offgases from the melamine synthesis section in the condensation section.
Thus, conventional methods for treating offgases coming from a melamine synthesis reactor are washed in a washing unit or scrubber unit using liquid urea. The offgas washing allows for a purification of the offgases from gaseous melamine and side products and leave the head of the washing unit usually with a temperature between 195 to 205° and a pressure above 10 MPa. These offgases are then transferred directly, i.e. in gaseous form, or after their condensation to an aqueous carbamate solution to the high pressure part of the urea plant, wherein the offgases are used for urea synthesis.
Although the offgases leaving the offgas washing unit comprise practically only ammonia and carbon dioxide and are therefore dry, undesired corrosion phenomena are recognized in the offgas pipeline connecting the washing unit of the melamine plant to the urea plant. These undesired corrosion phenomena may be due to small traces of water, which favour the condensation of the offgases to carbamate. Another reason for the corrosion phenomena may be that the condensation of the offgases is caused by bad insulation which in turn causes a decrease of the temperature of the offgases and subsequently in combination with traces of water, a condensation of the offgases.
It is known that carbamate is a high corrosive liquid (Nitrogen, September/October 1996, Nr. 223, pages 39 to 48). In order to avoid or reduce the undesired corrosion caused by the carbamate liquid it has been suggested to flush the synthesis equipment, in particular, the pipelines being in contact with the carbamate solution with passivating air. Passivating air may be additionally enriched with oxygen. The passivating air promotes the formation of stable oxide layers on the inside surfaces of the equipment and prevents therefore corrosion. The passivating air is for instance introduced into the upper area of the urea scrubber and flushes therefore also the offgas pipelines.
A further reason for the corrosion detected in offgas pipelines can be seen in that solid particles carried over are deposited locally in the offgas pipeline, for instance at locations where the surface is uneven due to welding seams. These deposits can prevent the contact of the metal surface with the passivating air; thereby corrosion can occur at these locations.
The offgas pipeline for transferring the offgases from the melamine plant to the urea plant is the central connecting pipeline between both plants and may have a length of up to several hundred meters. For instance, it is not unusual that the connecting pipeline in particular in older plants can have a length between 200 to 300 m, whereas in newer plants the length of the pipeline can be reduced to about 10 or 20 m. However, due to the strong corrosion problems encountered in these offgas pipelines it is necessary to shut down the complete plant complex in order to carry out the necessary repair. This is however cost intensive and therefore not desirable.
It is also known that the corrosion phenomenon increases with increasing temperature. Therefore, a possible approach for reducing the corrosion in the offgas pipeline after or downstream of the urea scrubber may be to decrease the temperature of the offgases coming from the scrubber. This could be for instance done by operating the offgas scrubber on a lower temperature level. The disadvantage of this approach is, however, that the offgas heat cannot be used as usual for the pre-heating of the urea melt entering the melamine reactor.
Another approach could be to feed water into the offgas pipeline. This, however, would be contra productive for the urea synthesis, since additional water negatively influences the synthesis rate of urea and would lead to a higher consumption of operating material like steam. Furthermore, it would be necessary to remove the water from the offgases before entering the urea plant.